We want the discriminant to be negative, so we want
b2 - 4(1)(9) < 0
b2 - 36 < 0
b2 < 36
This is true when...
| b | < 6
Which is true when...
b < 6 and b > -6
So b is any number in the interval (-6, 6)
(The endpoints are not included because that makes the discriminant equal to zero, which gives only one real root to the original polynonmial.)
Check: https://www.desmos.com/calculator/q4gcahv61l
Try moving the slider for b and notice that only and for all b values in the interval (-6, 6) , the graph is entirely above the x-axis.